There are several acceptable reasons i can think off, many of which have already been discussed, but some of the reasons i can think off have not yet been brought up (at least, i quickly scanned the thread, i did not read every single reply in detail):

• Anonimity: i think it's perfectly fine to have an account that's completely anonymous, and a second account you use to do business, participate in raffles, attach to domain names,...)
• Security: an account with a weak password for public pc's, cellphones, travel Wifi's,... and a main account with a strong password
• Brand recognition: i have a client with multiple accounts, one for each big project he starts... Once again, perfectly fine by me
• Name reservation: i think it would be perfectly fine to reserve your name (your real name or business name), even if you don't use the account... Just to avoid scammers to use your name to scam others
• Unwanted, but legal behaviour: there are some kinds of behaviour that aren't really likeable, like cursing... Maybe you feel like ridiculing somebody but don't want other's to shun you, so you use an alt account... It's a bit sketchy, but i'd still call this borderline acceptable

Offcourse, alt accounts for scamming, spamming, self bumping, cheating,... should be kicked off the forum. However, there are plenty of good reasons to have an alt imho